Problem persisting collection of interfaces in JDO/Datanucleus. "unable to assign an object of type.."

I am getting below error whilst trying to persist an object that has a collection of interfaces which I want to hold a couple of different types of objects. Seems to be happening almost randomly. Sometimes after restarting it works ok ( I might be doing something wrong though).

class CommentList {

@Persistent
@Join
ArrayList<IComment> = new ArrayList<IComment>();

}

somewhere else...

CommentList cl = new CommentList();

cl.addComment( new SimpleComment() );
cl.addComment( new SpecialComment() );

repo.persist( cl );

I can see the join table has been created in my DB along with ID fields for each of the Implementation classes of IComment.

SimpleComment and SpecialComment implement IComment. If I just add a SimpleComment it works fine. As soon as I start trying to add other types of objects I start to get the errors.

error im getting

java.lang.ClassCastException: Field "com.myapp.model.CommentList.comments" is a reference field (interface/Object) of type com.myapp.behaviours.IComment but DataNucleus is unable to assign an object of type "com.myapp.model.ShortComment" to this field. You can only assign this field to a type specified by the "implementation-classes" extension attribute.
at org.datanucleus.store.mapped.mapping.MultiMapping.setObject(MultiMapping.java:220)
at org.datanucleus.store.mapped.mapping.ReferenceMapping.setObject(ReferenceMapp开发者_Python百科ing.java:526)
at org.datanucleus.store.mapped.mapping.MultiMapping.setObject(MultiMapping.java:200)
at org.datanucleus.store.rdbms.scostore.BackingStoreHelper.populateElementInStatement(BackingStoreHelpe
r.java:135)
at org.datanucleus.store.rdbms.scostore.RDBMSJoinListStoreSpecialization.internalAdd(RDBMSJoinListStore
Specialization.java:443)
at org.datanucleus.store.mapped.scostore.JoinListStore.internalAdd(JoinListStore.java:233)

When it does save, if I restart the server and try to query for a list of the comments, I get null values returned.

I'm using mysql backend - if I switch to db4o it works fine.

Please let me know if any info would be useful.

If you have any idea where I might be going wrong or can provide some sample code for persisting collection of different objects implementing the same interface that would be appreciated.

Thanks for any help.

Tom

When I used interfaces I just enabled dynamicSchemaUpdates (some persistence property with a name like that) and FK's are added when needed. The log gives all SQL I think

I fixed this by specifying

<extension implemention-classes="SimpleComment SpecialComment"/>

for the field cl in my pacakge.jdo.